What is pH?

pH, or power of hydrogen, is a logarithmic scale used to measure how acidic or basic a solution is based on the hydrogen ion (H+) concentration. The concentration of hydrogen ions in a solution determines the acidity or alkalinity of a solution. In acidic solutions, the concentration of hydrogen ions is greater than the concentration of hydroxide ions, whereas in alkaline solutions, the concentration of hydrogen ions is lower than the concentration of hydroxide ions.

The Arrhenius theory of acids and bases introduced in 1884 is a simple theory that explains the classification of compounds as acids or bases based on the kind of ions formed when added to water. It states that when an acid is added to water, it donates a hydrogen ion (H+) to water to form H3O+. The higher the concentration of H3O+ (or H+) in a solution, the more acidic the solution. In contrast, a base substance generates OH- ions. The higher the concentration of hydroxide ions (OH-) in a solution, the more basic the solution is. The following equation is used to determine the pH value based on the concentration of the hydrogen ions.

pH = -log[H+]

The pH scale ranges from 0 to 14. A pH value lower than 7 indicates that a solution is acidic, a pH value equal to 7 is neutral, and a pH value higher that 7 indicates that a solution is basic. The lower the pH, the stronger the acidity, and the higher the pH, the stronger the alkalinity.

There are three main ways to measure the pH level of a solution.

A paper strip coated in a pH indicating dye can be used with a pH color chart

A pH testing kit can be used for liquids with a pH color chart

A digital pH meter can be used to automatically obtain the pH level of the solution. It is also the most accurate

How pH Affects Plant Nutrient Absorption, Fish and Bacteria

Managing the pH level is critical in aquaponics for the health of the entire system, as with hydroponics and soil gardening. However, managing the pH level is more challenging with aquaponic ecosystems as it constitutes three main living organisms – plants, fish and bacteria, all of which have different pH level preferences.

As the pH of the soil affects the dispersal of some plant nutrients, it greatly influences the availability of nutrients to plants and the nutrient absorption by plants. Thus, the soil pH also impacts the plant growth. Most plant nutrients are available within the range 6.5 to 7.5, so the ideal soil pH is close to neutral. Extreme soil pH levels can be detrimental to plant. In very alkaline soil, micronutrients such as zinc and copper may become chemically unavailable. In very acidic soil, micronutrients such as calcium, magnesium and phosphorus are not absorbed as the availability decreases. Thus, it is important to maintain a pH level within the optimum range.

The optimum pH range for fish depends on the type of fish being used for the aquaponic system. While saltwater fish prefer an alkaline pH of 8.0 or above, freshwater fish thrive in a slightly acidic pH ranging from 5.5 to 7.5. Thus, the optimum pH range for fish is considered to be 6.5 to 9.0. Outside this optimum pH range, if the pH is too high or too low, the fish can become stressed and there is an increased risk of dying. The pH of the water can also affect how soluble and toxic of chemicals and heavy metals the water is which can heavily impact the wellbeing of the fish.

Bacteria are extremely adaptable in general. However, every microorganism has an optimum pH value for growth. The optimum pH range for most bacteria growth is 6.5 to 7.0. However, some bacteria may thrive in very acidic conditions, some bacteria may be able to tolerate pH values as low as 1. Even though these bacteria may thrive in very acidic conditions, their internal pH is much closer to neutral.

As plants generally prefer a slightly acidic environment, while fish and bacteria prefer a slightly alkaline condition, a pH within range of 6.5 - 7.5 is usually maintained in aquaponic systems.

Carbonate Buffer and Water Hardness

The pH level of the water in the aquaponic system is affected by the water hardness which determines the buffering capacity of water. The buffering capacity refers to the ability of the water to maintain a stable pH despite small changes such as the addition of acids or bases. If you are not using distilled water, there will likely be some dissolved mineral salts present which will impact the pH level of the water. The term water hardness is used to describe the concentration of these dissolved mineral salts in the water. The higher the concentration of the minerals, the harder the water is and vice versa.

There are two main types of water hardness – carbonate hardness (KH) and general hardness (GH). Carbonate hardness refers to the water’s buffering capacity or alkalinity, whereas the general hardness refers to the concentration of calcium and magnesium ions. The carbonate hardness plays a bigger role in determining the buffer potential of the aquaponic system to stabilize the pH. Buffers are essential to prevent a dramatic or abrupt change in the pH level, which can be fatal to the fish in the system. Buffers also play a key role in maintaining the health of bacteria in the system. A very low carbonate hardness could result in an extremely low pH which will kill off most bacteria.

For an aquaponic system, the optimal KH level is anything above 4.0 which ensures that a high buffer potential is maintained for a more stabilized pH. The KH level of an aquaponic system can be measured using a designated water test kit.

To increase the KH level of a system if it falls below the optimal level of 4.0 dKH, you can add potassium bi-carbonate to the system at a rate of 2 ½ teaspoons per 100 gallons of water to increase the dKH level.

What Causes pH in Aquaponic Systems to Fluctuate

It is important to be aware of the factors that result in the fluctuation of the pH of your system to maintain a near-neutral level. There are several external factors that may result in an increase or decrease in the pH level of an aquaponic system.

An increase in the pH is usually due to carbonate buildup in the system which may be due to the water used being hard. The grow media being used may also contribute, as well as the materials used to build the take or beds including limestone and concrete. Although a high pH is common during the initial startup of the system, a well-established bacteria colony can eventually reduce the pH by preventing the buildup of carbonate via the nitrification process. A very high pH can result in poor plant growth and poor fruit and flower development.

A decrease in the pH is commonly due to the nitrification process of converting fish waste into plant food which increases the acidity of the water in the system. A decrease in pH may also due to the material used to construct the aquaponic system or the type of plants being grown. A low pH should be raised to avoid problems with the fish and bacteria in the system.

How to Adjust pH in Aquaponic Systems

A buffer can help regulate the pH of the water in the aquaponic system, however you may have to intervene when the pH level becomes too high or too low.

When the pH is too low, you will have to increase the alkalinity of the water to raise the pH level. This can be done by combining calcium carbonate and potassium carbonate in equal amounts and adding the mixture to the water. Carbonates are preferred as they are not caustic and will add strength to the carbonate buffer. You can also add simple hydroxides such as sodium hydroxide to raise the pH level of the water.

When the pH is too high, you will have to increase the acidity of the water to reduce the pH level. This can be done by adding phosphoric acid, which is safe and effective. This also adds phosphate into the system which will further benefit the plants in terms of nutrients. A reverse osmosis (RO) filter can also be used to keep the pH from getting too high. RO filters can remove the carbonates from the water, resulting in very pure, carbonate-free water with a reduced the pH.

Additional Tips

To lower the pH, never use vinegar, which is too weak, or citric acid which is anti-bacterial and acts as herbicide.

Consistently monitor and adjust the pH level to ensure that your aquaponic system always functions properly

Make sure to make any adjustments to the pH level gradually to avoid shocking any living elements of your system

Conclusion

Managing correct pH is critical in maintaining a well-functioning aquaponics system. Every living thing in the system, whether it be the fish, the plants, or the bacteria, has their own ideal pH range, but a sweet spot of 6.5 - 7.5 should work just fine. Maintain appropriate KH at above 4.0 and add potassium bi-carbonate if necessary.

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